1. Field of the Invention
The present invention relates to a method and apparatus for feedback control of an air-fuel ratio in an internal combustion engine having two air-fuel ratio sensors upstream and downstream of a catalyst converter disposed within an exhaust gas passage.
2. Description of the Related Art
Generally, in a feedback control of the air-fuel ratio in a single air-fuel ratio sensor (O.sub.2 sensor) system, a base fuel amount TAUP is calculated in accordance with the detected intake air amount and detected engine speed, and the base fuel amount TAUP is corrected by an air-fuel ratio correction coefficient FAF which is calculated in accordance with the output signal of an air-fuel ratio sensor (for example, an O.sub.2 sensor) for detecting the concentration of a specific component such as the oxygen component in the exhaust gas. Thus, an actual fuel amount is controlled in accordance with the corrected fuel amount. The above-mentioned process is repeated so that the air-fuel ratio of the engine is brought close to a stoichiometric air-fuel ratio. According to this feedback control, the center of the controlled air-fuel ratio can be within a very small range of air-fuel ratios around the stoichiometric ratio required for three-way reducing and oxidizing catalysts (catalyst converter) which can remove three pollutants CO, HC, and NO.sub.x simultaneously from the exhaust gas.
In the above-mentioned O.sub.2 sensor system where the O.sub.2 sensor is disposed at a location near the concentration portion of an exhaust manifold, i.e., upstream of the catalyst converter, the accuracy of the controlled air-fuel ratio is affected by individual differences in the characteristics of the parts of the engine, such as the O.sub.2 sensor, the fuel injection valves, the exhaust gas recirculation (EGR) valve, the valve lifters, individual changes due to the aging of these parts, environmental changes, and the like. That is, if the characteristics of the O.sub.2 sensor fluctuate, or if the uniformity of the exhaust gas fluctuates, the accuracy of the air-fuel ratio correction amount FAF is also fluctuated, thereby causing fluctuations in the controlled air-fuel ratio.
To compensate for the fluctuation of the controlled air-fuel ratio, double O.sub.2 sensor systems have been suggested (see: Japanese Unexamined Patent Publication (Kokai) Nos. 55-37562, 58-48755, and 58-72647). In such a double O.sub.2 sensor system, another O.sub.2 sensor is provided downstream of the catalyst converter, and thus another air-fuel ratio operation is carried out by correcting delay time parameters of an air-fuel ratio operation of the upstream-side O.sub.2 sensor with the output of the downstream-side O.sub.2 sensor. That is, in a single O.sub.2 sensor system, the switching of the output of the upstream-side O.sub.2 sensor from the rich side to the lean side or vice versa is delayed for a definite time period thereby stabilizing the feedback control, but such a definite time period is variable in the above-mentioned double O.sub.2 sensor system. In this double O.sub.2 sensor system, although the downstream-side O.sub.2 sensor has lower response speed characteristics when compared with the upstream-side O.sub.2 sensor, the downstream-side O.sub.2 sensor has an advantage in that the output fluctuation characteristics are small when compared with those of the upstream-side O.sub.2 sensor, for the following reasons:
(1) On the downstream side of the catalyst converter, the temperature of the exhaust gas is low, so that the downstream-side O.sub.2 sensor is not affected by a high temperature exhaust gas.
(2) On the downstream side of the catalyst converter, although various kinds of pollutants are trapped in the catalyst converter, these pollutants have little affect on the downstream-side O.sub.2 sensor.
(3) On the downstream side of the catalyst converter, the exhaust gas is mixed so that the concentration of oxygen in the exhaust gas is approximately in the equilibrium state.
Therefore, according to the double O.sub.2 sensor system, the fluctuation of the output of the upstreamside O.sub.2 sensor is compensated for by a feedback control using the output of the downstream-side O.sub.2 sensor. That is, even when the upstream-side O.sub.2 sensor is deteriorated, the emissions such as HC, CO, and NO.sub.x can be minimized by the correction of the delay time parameters by the output of the downstream-side O.sub.2 sensor. Actually, as illustrated in FIG. 1, in the worst case, the deterioration of the output characteristics of the O.sub.2 sensor in a single O.sub.2 sensor system directly effects a deterioration in the emission characteristics. On the other hand, in a double O.sub.2 sensor system, even when the output characteristics of the upstream-side O.sub.2 sensor are deteriorated, the emission characteristics are not deteriorated. That is, in a double O.sub.2 sensor system, even if only the output characteristics of the downstream-side O.sub.2 are stable, good emission characteristics are still obtained.
In the above-mentioned double O.sub.2 sensor system, however, when the upstream-side O.sub.2 sensor is deteriorated so that the controlled center thereof is shifted, one of the delay time parameters corrected by the downstream-side O.sub.2 sensor is too large, thereby reducing the response speed (i.e., the control frequency), thus reducing the accuracy of the feedback control. For example, as shown in FIG. 2A, when the upstream-side O.sub.2 sensor, which generates an output voltage V.sub.1, is only slightly deteriorated, a rich time parameter TDR, for which the switching of the output of the upstream-side O.sub.2 sensor from the lean side to the rich side is delayed, is set at 32 ms, and a lean time parameter TDL, for which the switching of the output of the upstream-side O.sub.2 sensor from the rich side to the lean side is delayed, is also set at 32 ms, so that the frequency of the feedback control is about 1.3 Hz. Contrary to this, when the upstream-side O.sub.2 sensor is deteriorated, the rich time parameter TDR is set at 8 ms and the lean time parameter TDL is set at 256 ms, so that the frequency of the feedback control is about 0.93 Hz. This means that the response characteristics are reduced by about 30%, and surging may be generated. In FIGS. 2A and 2B, FAF designates an air-fuel ratio correction amount which will be explained later.
Note that, in order to avoid the reduction of the response speed, a maximum limit is imposed on the delay time parameters corrected by the output of the downstream-side O.sub.2 sensor (see: FIG. 4 of Japanese Unexamined Patent Publication (Kokai) No. 58-72647). In this case, when one of the delay time parameters reaches such a maximum limit, the feedback control by the downstream-side O.sub.2 sensor is substantially suspended, i.e., a double O.sub.2 sensor system is suspended.